1. Field of the Invention
This invention pertains to a color television system including a device for the encoding of color television signals and a device for the decoding of color television signals. In the decoder, the high-frequency of the color television signal of the first and second fields are added thereby eliminating cross-luminance and the demodulated, lower frequency color difference signals are added thereby eliminating cross-color.
2. Description of the Prior Art
When color television was initially introduced, the luminance and chrominance signals were transmitted using the technique of frequency interlacing in order to ensure compatibility with previously existing television transmission systems and to facilitate transmission of the color information within the existing communication channels having a cutoff frequency of typically 5 megahertz. Although economical use of the channel width was indeed achieved with frequency interlacing, cross-talk interference occurs during decoding of the received television signals which sometimes significantly impairs the picture quality. Cross-talk from the luminance signal into the chrominance channel--this interference is referred to as cross-color--causes high-frequency horizontal luminance structures to suffer interference which appears as traveling red/green or blue/yellow wave patterns. Cross-talk from the chrominance signal into the luminance channel, called cross-luminance, appears as lines of achromatic dots in colored areas and along colored edges. Both types of cross-talk interference flicker in stationary images at a frequency of 6.25 Hz and 18.75 Hz.
To reduce this cross-talk interference, the magazine Radio Mentor, Volume 12, 1969, pages 847 through 851, published a description of a system for encoding of color television signals under the PAL standard, in which analog signal processing is performed using a disconnectable notch filter placed in the luminance channel. It is indeed possible to prevent the luminance component from being incorrectly evaluated on the receiving side as chrominance signals during demodulation of the chrominance subcarrier through the use of this notch filter. However, this technique involves a reduction of the horizontal resolution of the luminance signal to a value below 4 megahertz.
In the course of further development, digital signal processing techniques in the PAL encoder were investigated, in which the luminance and chrominance signals were subjected to comb filtering before combination. In the three-dimensional spectral representation of the image, comb filtering corresponds to a two or three-dimensional band separation which--with appropriate digital signal processing in the PAL decoder--can produce a picture with little cross-talk interference.
The magazine Fernseh- und Kinotechnik, Volume 39, Number 3, 1985, pages 123 through 135, described a PAL encoder and decoder in which a three-dimensional band limitation of luminance and chrominance signals is undertaken in such a way that the luminance and chrominance signals occupy different frequency ranges. This solution was further developed in the subject of the previously unpublished German patent application P38 07 248.3, in which separate three-dimensional filtering of the color difference signals permits transmission of each of these color difference signals with optimal resolution. In order to take into account the realities of the physiology of vision with regard to the color television standard, different pass bands in the filter were selected in this system for vertical spatial frequency and frame rate, for both color difference signals which are band-limited in the horizontal spatial frequency. For a color television signal conforming to the PAL standard, it is possible in this way to take into account that the human eye is significantly less sensitive to blurring of motion with respect to the color difference signal U than it is to blurring of motion with respect to the color difference signal V.
Additional solutions with three-dimensional filtering in the encoder and decoder are explained in detail in the magazine Fernseh- und Kinotechnik, Volume 42, Number 9, 1988, pages 403 through 422 and in the magazine Funkschau, No. 1, 1989, pages 46 through 50.
Common to all of the above-identified solutions is that great circuit complexity in the encoder and decoder is required in order to eliminate the cross-talk interference, which is associated with a loss in the resolution of details, i.e., reduced resolution for luminance and chrominance.